Humans in many societies cooperate in economic experiments at much higher levels than would be expected if their goal was maximizing economic returns even when interactions are anonymous and one-shot. This is a puzzle because paying a cost to benefit another player in one-shot interactions has no direct benefit to the cooperator. This paper explores the logic of two competing evolutionary hypotheses to explain this behavior. The "norm psychology" hypothesis holds that a player's choice of strategy is heavily influenced by socially-learned cultural norms. Its premise is that over the course of human evolutionary history, cultural norms varied considerably across human societies and through a process of gene-culture co-evolution, humans evolved mechanisms to learn and adopt the norms of their particular society. The "evolutionary mismatch" hypothesis holds that pro-social preferences evolved genetically in our hunter-gatherer past where one-shot anonymous interactions were rare and these evolved "protocols" for cooperation are misapplied in modern, laboratory, conditions. I compare these hypotheses by adopting a well known model of the mismatch hypothesis. I show that the cooperation generated by the model is based on a flawed assumption - that the best thing to do is cooperate in a repeated game. I show that repeated games generate a great diversity of behavioral equilibria, in support of the norm psychology hypothesis's premise. When interaction is repeated, adopting local norms is a more evolutionarily successful strategy than automatically cooperating. If various groups are at different behavioral equilibria, then cultural selection between groups tends to select for cooperative behavior.

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